1. Field of the Invention
The present invention relates to a liquid crystal display apparatus, in particular, to an active-matrix type liquid crystal display apparatus.
2. Description of the Related Art
Liquid crystal displays are used as display apparatuses of low-power consumption, small-thickness, and light-weight type. Particularly, in active-matrix type liquid crystal displays that use thin film transistors for individual pixels, even if the number of pixels is increased, moving pictures can be displayed with a high picture quality including high contrast and high response. Thus, the liquid crystal displays have been used in computers as well as TV sets as essential components.
In recent years, portable computers have been widely used because they can receive, edit, and send information anytime and anyplace. In this situation, it is very important to reduce the power consumption of the apparatuses so as to prolong the operation time thereof. To reduce the power consumption, various techniques have been employed. However, the power consumption of liquid crystal displays needs to be further reduced.
FIG. 21 is a schematic diagram showing a structure of a display portion of a conventional active-matrix type liquid crystal display apparatus. FIG. 22 is a schematic diagram showing a structure of a circuit of a pixel portion of the conventional liquid crystal display apparatus.
One transistor is disposed at each pixel. With a voltage applied to a gate line, the transistor is turned on. The voltage of the signal line at the point is applied to the liquid crystal. When another pixel is driven, the gate voltage is decreased so as to turn off the transistor. Thus, the electric charge is stored in the liquid crystal and a capacitor.
Normally, an AC voltage must be applied to the liquid crystal. In addition, to suppress the screen's flickering, the liquid crystal should be driven at a frequency so that the eyes of human beings cannot detect the AC voltage (for example, at 60 Hz). Thus, even if a picture does not move (namely, a still picture), pulses must be always applied to the signal line and gate line.
The driving circuit of the signal line is composed of a shift register, a sample hold circuit, and an output buffer. Normally, so as to successively sample a display signal in a predetermined interval, the clock frequency is obtained as the reciprocal of the scanning interval divided by the number of signal lines. In the case of a display having 1280.times.1024 pixels, the clock frequency becomes around 80 MHz. The power consumption of the driving circuit is proportional to the clock frequency. In addition, the power consumption of the liquid crystal panel is obtained by (applied voltage).sup.2 .times.(capacitance).times.(frequency). The frequency of the signal change of the signal line is the reciprocal of the scanning interval. Thus, the frequency of the signal change is around 61 kHz. Consequently, since the power consumption is proportional to the frequency, as long as the refresh rate of the liquid crystal is fixed, it is difficult to reduce the power consumption.
For example, the power consumption of a LCD with a diagonal length of 10.4 inches in VGA (640.times.480 pixels) is around 1 W. Thus, the LCD cannot be used for a display apparatus of a portable information unit for a long time. In addition, liquid crystal display apparatuses that have high resolutions corresponding to screen information tend to require high power consumption. Thus, it is very important to reduce the power consumption of liquid crystal display apparatuses.
On the other hand, a known technique uses a ferroelectric liquid crystal that has a memory property for allowing the refresh rate and thereby the power consumption to decrease. However, when the liquid crystal has the memory property, gradation display cannot be performed. Thus, in this case, the number of display colors is markedly decreased. Consequently, the display quality of the liquid crystal display apparatus is markedly deteriorated.
The present invention is made from the above-described point of view. An object of the present invention is to provide a liquid crystal display apparatus that can markedly reduce the power consumption of the driving circuit without adversely affecting the display quality. Another object of the present invention is to provide a liquid crystal display apparatus that has low power consumption that allows a portable information unit used with the apparatus to operate for a long time.